Your motherboard has 2 roads to each RAM slot.
Each road is 64 lanes wide.
That 512Mb module has 4 roads.
Each road is 32 lanes wide.
The only connections allowed are one road to one road and one lane to one lane.
1/2 or your 512Mb isn't there because there aren't any roads on the motherboard to it connect to.
1/2 of the 256MB that is left isn't there because it only has 1/2 as many lanes as are required.
.
